The present invention relates to a tubular casing for foodstuffs, in particular for use as a sausage casing, to a process for its manufacture, and to its use as a sausage casing for boiled sausages and cooked sausages, in particular sausages of the liver sausage type.
Tubular casings made from a web, for example, of regenerated cellulose, which are manufactured by folding the web along the longitudinal axis and gluing the overlapping edge zones running parallel to the longitudinal axis are already known (see, e.g., U.S. Pat. Nos. 2,148,884, 2,226,442, 2,685,769, 2,685,770, 2,757,495 and 2,773,773). The gluing operations hitherto described comprise either the use of solvents with which the surface of the web is incipiently dissolved and tackified, whereupon an "adhesive" is produced in situ on the surface of the web, or the use of webs which are coated with a thermoplastic resin or are composed of a thermoplastic material which can be sealed when heat and pressure are applied.
Thus, U.S. Pat. No. 2,653,432 describes a process for bonding the overlapping edges of a film which, for example, can also be composed of regenerated cellulose and, if appropriate, includes a fiber reinforcement, by means of a strip of thermoplastic material which is located between the overlapping edges and becomes tacky on heating. Polyvinylidene chloride is mentioned as an example of this thermoplastic material. This known tubular casing can also have a moisture-tight coating on the outside, the overlapping edges then being kept free of coating and the remaining surface being kept free of thermoplastic material. The casing material can be pre-stretched, preferably in the longitudinal direction.
This old process, which has been known for about 30 years, is hardly used any longer, particularly because an overlapping seam formed by sealing of the superposed edge zones is not sufficiently strong and is also completely unsatisfactory for reasons of appearance. In the meantime, sausage casings have exclusively been manufactured from seamless material, although some disadvantages must be accepted in the case of these seamless tubes.
Seamless tubes of fiber-reinforced regenerated cellulose are usually manufactured by the following essential process steps. A web of non-woven fiber is formed to give the shape of a tube, the edges of which overlap and are glued with viscose. After one-sided or two-sided coating with viscose from annular slot dies, the tube is coagulated, regenerated to give cellulose hydrate gel, and dried. During this process, the tube is supported by the gas mixture formed during the coagulation and regeneration. During drying, it is supported by an enclosed volume of air which has been introduced and, if appropriate, it is stretched in the longitudinal and transverse directions in order to improve the mechanical properties.
Disadvantageously, weakened zones, so-called spinning edges, which run parallel to the longitudinal axis in the edge region appear, for example, on the edges of the gel tube laid flat. Moreover, the stretching process also has disadvantages.
Neither the internal pressure of the process gases nor the internal pressure during drying can be controlled with sufficient accuracy, so that the diameter of the tubes cannot be kept constant within narrow limits. A further disadvantage is that, when tubes are stretched by means of an expanding gas, the ratio of the stretching forces acting in the tangential and axial directions is fixed, so that, for this reason, the mechanical properties of the seamless tube are also fixed, in particular the shrinking properties and the strength in the longitudinal and transverse directions.
The internal coating which is required for the use of the tube as a sausage casing for boiling sausages and cooked sausages and which is composed, for example, of a material which is impermeable to water and water vapor, is introduced in solution or dispersion into the interior of the seamless tube. This is expensive in terms of process engineering and can be troublesome. The solvent or dispersing agent must, for example, in a process step which takes a long time, diffuse through the wall of the tube to the outside. Moreover, the film formation on the inner surface of the tube is difficult to control. Finally, the mechanical properties which have been established at great cost during the manufacture of the tube are frequently disadvantageously changed by the coating process. Since the tubes must be laid flat immediately after coating, there is a considerable risk of the water vapor-impervious coating layer being damaged or weakened in the region of the squeezed edge. These coating defects have the result that the filling is spoiled, or becomes unattractive, due to loss of water or access of oxygen.